1. Field of the Invention
This invention relates to a photographic zoom lens, and more particularly to a zoom lens suitable to cover the range from standard to medium telephoto.
2. Description of the Prior Art
In recent years, zoom lenses for 35 mm still camera have been becoming more and more compact. In a zoom lens comprising a magnification changing system consisting of three lens units and a master lens system, an attempt to make the lens system compact is divided into two elements, that is, making the magnification changing system compact and making the master lens system compact. Where an attempt is made to make the magnification changing system compact, the power of each unit of the magnification changing system must be made strong and particularly, in order to reduce the amount of movement of the magnification changing lens unit, the negative power of the second unit which contributes chiefly to magnification change becomes strong and therefore, Petzval sum becomes greatly negative and curvature of image field is so much aggravated that it is difficult to correct it. Also, high-order spherical aberration (particularly at the telephoto end) is created or the symmetry of coma is destroyed or the fluctuation of lateral chromatic aberration by the angle of view is produced and thus, correction of aberrations becomes very difficult. Moreover, the fluctuation of these aberrations by zooming become great and this is not preferable. Thus, compaction of the magnification changing system is naturally limited. On the other hand, considering the compaction of the master system, the full length can be reduced by constructing the master system by two positive and negative units and making it into a teletype, but the necessity of allotting a strong negative power to the rearward unit arises and Petzval sum is greatly increased in the negative sense and therefore, this is also naturally limited. In a zoom lens having a magnification changing system comprising three positive, negative and negative units, the light beam after having left the negative third lens unit is a strong divergent light beam and moreover, where the angle of view of the standard lens is covered by the 35 mm format, if the telephoto ratio of the master lens system is reduced, spherical aberration, particularly high-order spherical aberration, astigmatism, curvature of image field, etc. are sharply increased and correction of aberrations becomes very difficult. If the master lens is made into a construction approximate to the symmetric type such as a Gauss type lens in order to correct this spherical aberration, the oblique light beam is greatly diverged particularly by the strong diverging action of the negative lens of the forward unit in the master lens system in a lens wherein a diaphragm is disposed forwardly of the master lens system and therefore, the diameter of the lens of the rearward unit in the master lens system is increased, and this has greatly hampered compaction of the lens system.